1. Field of the Invention
The present invention relates to a resin etching solution and an etching process.
2. Description of the Related Art
Polyimide resins which have excellent characteristics including heat resistance and electrical insulating properties are widely used in the field of electronics. They are used, for example, for flexible printed boards and TAB tapes.
Polyimide resins are used in various forms including films, sheets and coating agents; when used as films they are not always in simple form, but often have copper foils attached to their film surfaces with adhesives or are plated with copper sputtering-copper electrolytes, or are in the form of laminates, with the polyimide resin casted or coated on a copper foil.
Thus, a large number of such substrates are marketed by companies in the field. As representative examples of simple polyimide films there may be mentioned Kapton (tradename of DuPont Co., U.S.), Upilex S or R (tradename of Ube Industries, Ltd.) and Apical (tradename of Kanegafuchi Chemicals, K.K.).
As representative examples of products with polyimide resins cast molded on copper foils there may be mentioned Neoflex (tradename of Mitsui-Toatsu Chemicals, K.K.) and Espanex (tradename of Shinnittetsu Chemicals, K.K.).
A representative example of a coating agent is Torayneece (tradename of Toray, K.K.), and representative examples of polyimide bonding sheets are UPA (tradename of Ube Industries, Ltd.) and SPB (tradename of Shinnittetsu Chemicals, K.K.).
Among the polyimide resins used in these various forms, those polyimide films used as circuit board bases are provided with throughholes and device holes which are usually formed by chemical etching methods carried out using alkali-hydrazine-based etching solutions.
However, since such hydrazine-based etching solutions have toxicity (carcinogenicity) and short usable lives, great care must be taken in handling them, and thus the setting of ideal conditions for the etching has presented constant difficulties.
Also, in the case of panels which are laminates of polyimide films and copper foils, when etching is performed on the polyimide films using the patterned copper foils as masks, hydrazine-based etching solutions easily filter through interfaces with the copper foil masks and the polyimide films, and therefore peeling occurs between the copper foil and polyimide film before the throughholes (etching holes) are formed in the polyimide film by etching, resulting in a disadvantage in that it becomes difficult to form etching holes of the desired shape.
Another etching solution used is the one disclosed in Japanese Examined Patent Publication No. 60-14776, for example, which comprises urea and an alkali metal compound; however, this etching solution has a much slower etching rate and results in more destruction of the etching hole pattern (flawed shape and dimensions) than in the case described above, and when the etching temperature is increased for a higher etching rate the urea decomposes producing ammonia which has a strongly irritating odor, and resulting in environmental sanitation problems.
Furthermore, etching solutions comprising alkali metal compounds, phenolic solvents and water, as well as etching compositions prepared by adding either water or alcohol to alkali metal compounds, phenolic solvents and amine compounds, have been publicly known (being disclosed, for example, in Japanese Unexamined Patent Publication No. 5-301981).
However, even though all of these conventional etching solutions can be used for etching of polyimide films obtained by polycondensation of pyromellitic dianhydride and aromatic diamines (for example, the aforementioned Kapton and Apical), they have a crucial weakness in that they are poorly suitable for etching of polyimide films obtained by polycondensation of biphenyltetracarboxylic dianhydride and diaminobenzene, such as Upilex S (tradename of Ube Industries, Ltd.), which have excellent heat resistance, form stability and chemical resistance and are thus better suited for minute circuit pattern formation, compared to the former type of polyimide films.
For this reason, when miniature throughholes 200 microns or less in diameter are formed, for example, in double-layer polyimide films consisting of copper foils and Upilex S, such as Metaloyal (tradename of Toyo Metallizing, K.K.), the working must by necessity be accomplished with a costly excimer laser, and therefore a solution to this situation has been strongly desired.
In response to this need there have been proposed an etching solution comprising hydrazine monohydrate and potassium hydroxide, disclosed in Japanese Unexamined Patent Publication No. 3-101228 and an etching solution comprising sodium hydroxide, ethylenediamine, hydrazine monohydrate, aqueous dimethylamine and N,N-dimethylformamide, disclosed in Japanese Unexamined Patent Publication No. 5-202206.
However, none of these are fully satisfactory, and for example in the case of the aforementioned Metaloyal which has a pattern formed on the copper foil, when the copper foil is used as a mask for etching of the polyimide film, peeling occurs between the copper foil and polyimide film and therefore satisfactory etching holes (throughholes) without pattern destruction cannot be obtained.
Also, even though dimethylformamide solutions containing ethanolamine, disclosed for example in Japanese Unexamined Patent Publication No. 7-157560 under paragraph [0068], can be applied to polyimide resins which are soluble in organic solvents, they cannot be used for Kapton and Upilex, both of which are polyimide resins insoluble in organic solvents.